Far end crosstalk (FEXT) is a dominant source of disturbance in transmission systems where for example a plurality of communication lines is located close to each other. Such a situation may occur when the plurality of communication lines is provided in a so-called cable binder. To mitigate far end crosstalk, vectoring was developed. Vectoring is essentially a technique where signals transmitted via a plurality of communication lines are processed jointly, either before being transmitted or after being transmitted. In the first case, vectoring is also referred to as crosstalk precompensation, and in the latter case vectoring is also referred to as crosstalk cancellation.
For VDSL2 systems (very high bit rate digital subscriber line 2), vectoring was standardized in ITU-T recommendation G.993.5. According to this standard, training sequences are transmitted by modulating predefined sequences onto so-called synchronization symbols, also referred to as sync symbols. Sequences for different lines are selected to be orthogonal to each other. By evaluating error values (differences between received and transmitted sequences), crosstalk between lines can be estimated. Based on this estimation, crosstalk may be mitigated. For example, for crosstalk precompensation data transmitted via the line is pre-distorted by the data of every other line weighed by the respective crosstalk transfer function. A similar weighting is performed after signals are received in case of crosstalk cancellation.
However, for this mechanism, in particular the estimation of crosstalk coefficients, to be operational, devices involved (for example central office equipment and customer premises equipment) have to implement vectoring capabilities, e.g. comply with the above-mentioned standard. However, legacy devices exist which may for example be VDSL2 equipment not adapted to vectoring, i.e. not adapted to implement the mechanisms as specified in G.993.5. In some cases, communication lines (also referred to as legacy lines herein) coupled to such legacy devices are located close to other communication lines employing vectoring. Crosstalk from such legacy lines to vectored lines and vice versa cannot be cancelled or mitigated following the above-mentioned standard. Therefore, in conventional solutions the full benefit of vectoring may only be obtained in cable binders or similar arrangement of lines which all follow a common vectoring implementation, e.g. a vectoring standard. On the other hand, as legacy lines exist, it may be of interest to be able to take legacy lines into account when performing vectoring.